1. Field of the Invention
The present invention relates to a laser module comprising a semiconductor laser device enclosed in a hermetically sealed container.
2. Description of the Related Art
A laser module has been known comprising a semiconductor laser device, a collimating lens, a focusing lens, and an optical fiber positioned in a hermetically sealed container. In a laser module of this kind, a residual contaminant in a container adheres to optical components such as a laser end facet from which a laser light is emitted, a lens, and an optical fiber, resulting in deterioration of a characteristic of the laser module. As an example of such a contaminant, hydrocarbon compounds originating from an atmosphere in a manufacturing process can be listed. The hydrocarbon compounds are polymerized or decomposed by a laser light and deposited.
In order to solve this problem, various methods have been proposed as explained below. For example, U.S. Pat. No. 6,404,786 states that reduction in an amount of total hydrocarbon compounds in a container to 0.1% or lower is effective for preventing a decrease in the power of laser light whose wavelength is 400 nm or shorter. In this manner, deposition of hydrocarbon compounds on optical components or the like caused by photodecomposition of the hydrocarbons can be prevented. Furthermore, using a dry air as a sealed atmosphere has also been proposed, in expectation of removal of deposited hydrocarbons by photochemical reaction with oxygen in the sealed atmosphere.
Moreover, in U.S. Pat. No. 5,392,305, introduction of 100 ppm or more oxygen into a gas to be sealed in a container is described for decomposing an organic gas such as hydrocarbons, thereby adhesion of a contaminant to a laser end facet caused by photo-decomposition of the organic gas can be prevented.
In addition, Japanese Unexamined Patent Publication No. 11(1999)-87814 states that long-term reliability of a laser module can be assured by removing a contaminant such as oil in the module through defatting and cleaning thereof.
In U.S. Patent Laid-Open No. 20020090172, a laser module has been proposed, using a GaN semiconductor laser device whose emission wavelength is 350˜450 nm. In a laser module of this kind, a laser light of such short wavelength has high energy. Therefore, the probability is high that a material generated by polymerization or decomposition of hydrocarbons in the module adheres to a laser end facet and other optical components, which is especially problematic.
A hydrocarbon deposit generated by reaction of hydrocarbons with a laser light can be decomposed into CO2 and H2O in a gaseous atmosphere including a certain amount of oxygen, as has been described in U.S. Pat. No. 5,392,305. Therefore, the problem caused by the hydrocarbon deposit can be solved.
However, presence of silicon compounds is confirmed in such a deposit, in addition to the hydrocarbons, and it has been known that inclusion of oxygen in a laser atmosphere cannot decompose and remove the silicon compounds. A deposit comprising hydrocarbons and silicon compounds absorbs a laser light, which seriously damages temporal reliability of a laser module in continuous emission. Such silicon compounds are generated by photochemical reaction between a laser light and a gas of organic compounds including Si (hereinafter referred to as an organic silicon compound) such as a compound including the siloxane bond (Si—O—Si) and a silanol (—Si—OH) group. In addition, presence of oxygen in an atmosphere increases a speed of the reaction. The silicon compounds refer to organic or inorganic compounds having a structure including silicon. Therefore, inorganic SiOx and an organic silicon compound are included therein.
A source of the silicon compounds is mainly a gas generated from a silicone material used in a fabricating process of a laser module. The gas is present on the surface of components of the laser module in some cases, and a tiny amount of the gas is included in a sealed atmosphere in the case where the laser module is sealed. Complete removal of the gas component in the fabricating process cannot be realized by usage of an ordinary clean room or installation of a sealed-gas purifier. Therefore, a large amount of investment becomes necessary for managing the gas component. In addition, introduction of such a compound from an atmosphere in a fabricating process cannot be prevented in a defatting or cleaning process of oil disclosed in Japanese Unexamined Patent Publication No. 11(1999)-87814. Since a liquid organic substance is used in the cleaning process, management of impurities in a drying process is necessary. Furthermore, a cleaning agent needs to be selected that cannot dissolve an adhesive used for fixing a semiconductor laser device and an optical component. However, the characteristic of not dissolving an adhesive is in many cases contradictory to maintaining an ability to clean off the organic substance sticking to the component and the like.
The hydrocarbon compounds and the silicon compounds adhering to components in a laser module can be removed by devolatilization thereof through heating at a temperature of 200° C. or more, preferably at 300° C. or more. However, the heating takes several hours to several tens of hours. In addition, in the case where components in a module are fixed by an organic adhesive, thermal degradation of the adhesive leads to degradation of a mechanical characteristic thereof. Therefore, this method cannot be used.
Moreover, it is known that the method of introducing 100 ppm or more oxygen into a sealed gas, as has been described in U.S. Pat. No. 5,392,305, is not effective for a laser module having a semiconductor laser device whose emission wavelength is as short as 350˜450 nm. More specifically, in such a short-wavelength semiconductor laser device, dependency of a speed of temporal degradation on an oxygen concentration in an atmosphere becomes different from the case of a semiconductor laser device whose emission wavelength is in an infrared range. Therefore, no improvement is observed in association with an increase in oxygen concentration. In other words, in the case of a semiconductor laser device whose emission wavelength is in an infrared range, decomposition of an organic compound having hydrocarbons deposited on the surface of optical components along a laser light path, such as a laser end facet, optical components, and a fiber end in a module, can be facilitated by an increase in oxygen concentration, which leads to improvement in temporal reliability. On the other hand, in the case of a short-wavelength semiconductor laser device, reliability becomes lower when the oxygen concentration is 100 ppm. Deposition of silicon compounds, which becomes evident in 100 ppm or more oxygen concentration, is thought to cause this phenomenon. Although sealing oxygen up to a certain amount is usually necessary in order to prevent deposition of hydrocarbon compounds, deterioration of reliability of a system for emitting a laser light in an atmosphere including a large amount of hydrocarbon gas cannot be prevented, due to an increase in silicon compound deposit.